Method for determining parameters relevant to the print quality of a printed product

ABSTRACT

The invention provides a method for determining parameters relevant to the print quality of a printed product. A macroscopic photogram of a measuring field of the printed product is recorded using a camera having a macro lens. An actual value of a parameter relevant to the print quality is determined from the macroscopic photogram. The actual value is compared to a nominal value of the parameter relevant to the print quality. Whether the measuring field is printed with adequate quality is determined based on the comparison of the actual valve with the nominal value of the parameter relevant to the print quality.

FIELD OF THE INVENTION

The present invention relates to a method for determining parametersrelevant to the print quality of a printed product.

BACKGROUND OF THE INVENTION

Presently, measuring devices in the form of densitometers orcolorimetric measuring devices are used on printing machines in order todetermine parameters relevant to the print quality. These measuringdevices are used, in particular, for inspecting the measuring fields ofa print control strip of a printed product. Actual values of parametersrelevant to the printing process can be determined from the measuredvalues from the densitometer and/or the colorimetric measuring deviceand compared with predetermined nominal values for quality controlpurposes. Based on this comparison, the printing machine can be adjustedaccordingly, e.g., the ink can be adjusted.

Densitometers as well as colorimetric measuring devices utilize anintegral functional image of a measuring field to be inspected in orderto determine an actual value of a parameter relevant to the printquality of this measuring field. However, this does not take intoaccount whether the measuring field as such is neatly printed. If themeasuring field is not neatly or homogenously printed due toinsufficient contact pressure between the plate cylinder and the blanketcylinder or due to a defective or soiled rubber blanket, thedensitometer or the colorimetric measuring device does not deliver anexact actual value such that, for example, an ink control system basedon such an actual value can lead to inferior printing results.

BRIEF SUMMARY OF THE INVENTION

In view of the foregoing, a general object of the present invention isto develop a novel method for determining the parameters relevant to theprint quality of a printed product.

According to the invention, at least one macroscopic photogram of ameasuring field is recorded with the aid of a camera that features amacro lens. At least one actual value of at least one parameter relevantto the print quality is determined from the macroscopic photogram oreach of the macroscopic photograms recorded with the camera using animage processing method so as to determine if the measuring field isprinted with adequate quality.

The present invention involves inspecting measuring fields with the aidof a camera that features a macro lens, particularly a miniaturehigh-resolution camera, and recording corresponding macroscopicphotograms during this process. Actual values of parameters relevant tothe print quality can be determined from the recorded macroscopicphotograms using an image processing method in order to verify that themeasuring fields themselves are neatly printed. This method makes itpossible to examine full-tone measuring fields as well as halftonemeasuring fields with respect to a clean print image. The result of thisquality check, for example, can be used for deciding if the measuredvalues of a measuring field provided by a densitometer and/or acolorimetric measuring device are suitable for use in ink control.

If the measuring field consists of a full-tone measuring field for aprinting ink, an advantageous further aspect of the invention caninvolve determining an actual value for the full-tone measuring fieldfrom a gray scale value diagram of the complementary RGB-channel, namelyin the form of a uniformity distribution or a noise of the gray scalevalue over the measuring field.

If the measuring field consists of a halftone measuring field for aprinting ink, another advantageous aspect of the invention can involvedetermining an actual value for the halftone measuring field in the formof at least one geometric parameter for halftone dots of the halftonemeasuring field from the macroscopic photogram or a gray scale valuediagram of the complementary RGB-channel.

An exemplary embodiment of the invention is described in greater detailbelow with reference to the figures. However, the present invention isnot limited to this exemplary embodiment.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic drawing of an exemplary measuring device forcarrying out the method of the present invention.

FIG. 2 is a schematic flow chart of an exemplary embodiment of themethod of the present invention.

FIG. 3 is an exemplary macroscopic photogram of a full-tone measuringfield.

FIG. 4 is an exemplary gray scale value diagram of the full-tonemeasuring field or macroscopic photogram of FIG. 3.

FIG. 5 is another exemplary macroscopic photogram of a full-tonemeasuring field.

FIG. 6 is a gray scale value diagram of the full-tone measuring field ormacroscopic photogram of FIG. 5.

FIG. 7 is an exemplary macroscopic photogram of a halftone measuringfield.

FIG. 8 is a gray scale value diagram of the halftone measuring field ormacroscopic photogram of FIG. 7.

FIG. 9 is another exemplary macroscopic photogram of a halftonemeasuring field.

FIG. 10 is a gray scale value diagram of the halftone measuring field ormacroscopic photogram of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method for determining parametersrelevant to the print quality of a printed product, namely for verifyingwhether an inspected measuring field of the printed product is printedneatly and with adequate quality. Referring to FIG. 1 of the drawings,inspecting measuring fields of a print control strip 20, namely with theaid of a camera 21 that features a macro lens, is preferred. The camera21 can consist of a miniature high-resolution camera that records atleast one measuring photograph of the measuring fields of the printcontrol strip 20 to be inspected, namely a so-called macroscopicphotogram. The term “macroscopic photogram” refers to a measuringphotograph that is recorded with the aid of a camera featuring a macrolens a short distance from the measuring field to be inspected. Detailsof the inspected measuring field are magnified in the correspondingmacroscopic photogram similar to a magnifier. The magnification factorof the macro lens of the camera 21 is preferably between 20 and 50.

In FIG. 1, a print control strip 20 with a total of twelve measuringfields 22 is shown. Some measuring fields 22 are in the form of afull-tone measuring fields 22 a and other measuring fields are in theform of halftone measuring fields 22 b. The camera 21 featuring themacro lens can be mounted on a crossbeam and can be displaced relativeto the print control strip 20 as indicated by the double arrow 23 inorder to inspect each measuring field 22 thereof.

The camera 21 can be in the form of a separate component that can bedisplaced relative to the print control strip 20 independently of othercomponents in order to inspect the measuring fields 22. Alternatively,the camera 21 can be integrated into a measuring head that contains adensitometer and/or a colorimetric measuring device and in which thecamera can be displaced relative to the print control strip 20 togetherwith the densitometer and/or the colorimetric measuring device in orderto inspect the measuring fields 22.

According to the inventive method for determining the print quality of ameasuring field 22 with the aid of a camera 21, at least one macroscopicphotogram of the measuring field 22 is recorded in a first step 24.Subsequently, the macroscopic photogram or each macroscopic photogram isevaluated in a step 25 with the aid of an image processing method inorder to determine at least one actual value of at least one parameterof the inspected measuring field 22 that is relevant to the printquality. In the next step 26, each determined actual value is comparedwith a corresponding nominal value in order to verify that the measuringfield is printed or printed out with high or adequate quality. If it isdetermined that the measuring field is not printed out or printed withthe required quality, an alarm or error message can be generated at theprinting machine in a subsequent step 27 based on the comparison betweenthe actual value and the nominal value carried out in step 26.

The camera 21 can be in the form of a multi-bit camera, particularly an8-bit camera that inspects a measuring field 22 in the so-calledRGB-channels and preferably outputs a macroscopic photogram of themeasuring field 22 and a gray scale value diagram of the macroscopicphotogram or the measuring field 22 for each RGB-channel. In instancesin which an 8-bit camera is used, a total of 256 gray scale values canbe illustrated in the gray scale value diagram.

A macroscopic photogram of a measuring field in the form of thefull-tone measuring field 22 a and printed with a special printing inkis shown in FIG. 3. FIG. 4 is a gray scale value diagram 28 of themacroscopic photogram of FIG. 3 and therefore of the full-tone measuringfield 22 a that is made available by the camera 20 in the complementaryRGB-channel relative to the printing ink of the full-tone measuringfield 22 a. The image coordinates of the macroscopic photogram of thefull-tone measuring field 22 a are plotted on the X-coordinate and theY-coordinate of the gray scale value diagram 28. The gray scale valuesin the respective pixel of the macroscopic photogram of the full-tonemeasuring field 22 a are plotted on the Z-coordinate.

The gray scale value diagram 28 of FIG. 4 comprises a so-called invertedgray scale value diagram, in which a gray scale value of zerocorresponds to the maximum color value of the full-tone measuring field22 a such that deviations from this maximum color value appear in theform of peaks in the gray scale value diagram 28 of the macroscopicphotogram of the full-tone measuring field 22 a. An actual value for thefull-tone measuring field 22 a in the form of a uniformity distributionof the gray scale values over the image coordinates of the macroscopicphotogram of the full-tone measuring field 22 a or a noise of the grayscale value over the macroscopic photogram or the full-tone measuringfield 22 a can be determined from the gray scale value diagram 28. Itcan be concluded that a full-tone measuring field 22 a of adequate printquality is examined if the uniformity distribution or the noise isrespectively lower than the corresponding nominal value or limitingvalue as shown in the embodiment according to FIGS. 3 and 4.

In contrast, FIG. 6 is a gray scale value diagram 29 in the macroscopicphotogram of a full-tone measuring field 22 a according to FIG. 5, inwhich substantially larger deviations of the gray scale values areconcluded over the image coordinates of the macroscopic photogram of thefull-tone measuring field 22 a. In this case, the uniformitydistribution and the noise of the gray scale values are higher than thecorresponding nominal value or limiting value in numerous pixels fromwhich it can be determined that a full-tone measuring field 22 a ofinferior print quality is examined in this case.

It is therefore preferred to determine the uniformity distribution orthe noise of the gray scale values relative to a nominal value or alimiting value based on the image coordinates of the full-tone measuringfield 22 a or the image coordinates of the macroscopic photogram of thefull-tone measuring field in order to carry out a qualitative evaluationof the full-tone measuring field 22 a. In addition, how frequently or athow many pixels the gray scale value exceeds the nominal value orlimiting value of the uniformity distribution or the noise,respectively, is examined.

If substantial deviations from the nominal value or limiting value aredetected at numerous pixels, it can be concluded that a full-tonemeasuring field of inferior print quality is examined. However, if onlyslight deviations from the nominal value or limiting value are detectedat a relatively large number of pixels, it can be concluded that afull-tone measuring field of adequate print quality is examined.

The method of the present invention is also suitable for examininghalftone measuring fields. FIG. 7 is a macroscopic photogram of ahalftone measuring field 22 b in the region of six halftone dots. Thehalftone dots are in the form of round halftone dots in the illustratedembodiment. Any other shape of halftone dots may also be chosen in ahalftone measuring field 22 b instead of all round halftone dots. Inorder to evaluate the print quality of a halftone measuring field 22 b,at least one macroscopic photogram of the halftone measuring field 22 bis recorded, according to the invention, with the aid of a camera thatfeatures a macro lens. An actual value of at least one parameterrelevant to the print quality is determined from each macroscopicphotogram using an image processing method. With respect to the halftonemeasuring field 22 b, each actual value consists of a geometricparameter of the halftone dots of the halftone measuring field 22 b. Itcan be concluded that a halftone measuring field of adequate printquality is examined if each geometric parameter is lower than thecorresponding nominal value or limiting value, and that a halftonemeasuring field 22 b of inferior print quality is examined if eachgeometric parameter is higher than the corresponding nominal limitingvalue.

According to a first alternative embodiment of the present invention,the most frequent gray scale values are determined with the aid of agray scale value diagram 30 of the halftone measuring field 22 b usingan image processing method so as to define a geometric parameter forround halftone dots of a halftone measuring field 22 b. In this case,all image information that lies outside the most frequent gray scalevalues is filtered out of the macroscopic photogram.

Subsequently, a minimum halftone dot diameter D_(MIN) and a maximumhalftone dot diameter D_(MAX) are determined for each halftone dot byutilizing the correspondingly filtered macroscopic photogram of thehalftone measuring field 22 b. A first halftone dot deformation value isdetermined for each halftone dot from the minimum halftone dot diametersD_(MIN) and the maximum halftone dot diameters D_(MAX) by utilizing thefollowing formula:

${RPDW}_{1} = {\frac{D_{MAX} - D_{MIN}}{D_{MAX}}*100\%}$

wherein RPDW₁ is the first halftone dot deformation value of a halftonedot, D_(MAX) is the maximum halftone dot diameter of a halftone dot andD_(MIN) is the minimum halftone dot diameter of a halftone dot.

If the maximum halftone dot diameter D_(MAX) and the minimum halftonedot diameter D_(MIN) have approximately the same size and the halftonedot deformation value RPDW₁ of the halftone dots is consequentlyrelatively small as shown in the example of the filtered macroscopicphotogram of the halftone measuring field 22 b in FIGS. 7 and 8, it canbe concluded that the halftone dots of the halftone measuring field 22 bare round and printed with adequate quality.

However, if the minimum halftone dot diameter D_(MIN) and the maximumhalftone dot diameter D_(MAX) deviate significantly and the firsthalftone dot deformation value RPDW₁ is consequently relatively large asshown in the example of the filtered macroscopic photogram of thehalftone measuring field 22 b in FIGS. 9 and 10, it can be concludedthat the halftone dots have an inferior print quality and that doublingof the halftone dots has occurred. This means that the print quality ofthe halftone dots increases proportionally to the decrease in thedifference between the minimum and the maximum halftone dot diameter.

The difference between a halftone measuring field 22 b of adequate printquality according to FIG. 7 and a halftone measuring field 22 b ofinferior print quality according to FIG. 9 can also be determined basedon a comparison between the corresponding gray scale value diagrams 30and 31 according to FIGS. 8 and 9, which again consist of inverted grayscale value diagrams. For example, the gray scale value diagram 30according to FIG. 8 of a halftone measuring field 22 b of adequate printquality is characterized by round and defined transitions betweenadjacent halftone dots. In contrast, the gray scale value diagram 31 ofa halftone measuring field 22 b of inferior print quality showsundefined and unround transitions.

According to further aspect of the present invention, another geometricparameter in the form of a second halftone dot deformation value can bedetermined for each round halftone dot of a halftone measuring field inaddition to the above-mentioned first halftone dot deformation value,namely from a minimum surface of a halftone dot that is determined for afirst defined gray scale value range and from a maximum surface of ahalftone dot that is determined for a second defined gray scale valuerange. For this purpose, all pixels of the macroscopic photogram of thehalftone measuring field that lie outside the first gray scale valuerange are filtered out with the aid of an image processing method afterthe first gray scale value range is defined. The minimum surface of thehalftone dots of the halftone measuring field can then be calculatedwithin this first gray scale value range. Subsequently, the gray scalevalue range is increased and the maximum surface of the halftone dots isdetermined within this gray scale value range. The second halftone dotdeformation value is then calculated for each halftone dot from theminimum halftone dot surfaces and the maximum halftone dot surfaces byutilizing the following formula:

${RPDW}_{2} = {\frac{A_{MAX} - A_{MIN}}{A_{MIN}}*100\; \%}$

wherein RPDW₂ is the second halftone dot deformation value of a halftonedot, A_(MAX) is the maximum surface of a halftone dot and A_(MIN) is theminimum surface of a halftone dot.

If the difference in surface between the minimum halftone dot surfaceand the maximum halftone dot surface is small and the second halftonedot deformation value consequently is comparatively small, it can beconcluded that halftone dots of adequate print quality are examined andthat the halftone dots have sharp flanks or edges. However, if thedifference between the maximum halftone dot surface and the minimumhalftone dot surface is relatively large, it can be concluded thatbleeding of the halftone dots has occurred such that their edges orflanks are undefined.

The inventive method also makes it possible to detect smearing at thebeginning of the printing process by analyzing the edges of a printcontrol strip that was printed transverse to the transport direction ofthe material to be printed in the above-described fashion at thebeginning of the printing process.

The inventive method for determining whether measuring fields of aprinted product have an adequate print quality can be advantageouslycombined with a color control method in such a way that the actualvalues determined in the measuring fields with the aid of a densitometerand/or a colorimetric measuring device are only used for controlpurposes if it was determined beforehand that the measuring field has anadequate quality with the aid of the inventive method.

LIST OF REFERENCE SYMBOLS

-   20 Print control strip-   21 Camera-   22 Measuring field-   22 a Full-tone measuring field-   22 b Halftone measuring field-   23 Double arrow-   24 Step-   25 Step-   26 Step-   27 Step-   28 Gray scale value diagram-   29 Gray scale value diagram-   30 Gray scale value diagram-   31 Gray scale value diagram

1. A method for determining parameters relevant to the print quality ofa printed product comprising the steps of: recording a macroscopicphotogram of a measuring field of the printed product using a camerahaving a macro lens; determining an actual value of a parameter relevantto the print quality from the macroscopic photogram; comparing theactual value to a nominal value of the parameter relevant to the printquality; and determining whether the measuring field is printed withadequate quality based on the comparison of the actual valve with thenominal value of the parameter relevant to the print quality.
 2. Themethod according to claim 1 wherein the camera inspects the measuringfield in RGB-channels and records one macroscopic photogram and a grayscale value diagram of the measuring field for each RGB-channel.
 3. Themethod according to claim 2 wherein when the measuring field consists ofa full-tone measuring field for a printing ink and the actual value forthe full-tone measuring field is determined in the form of a uniformitydistribution value or a noise of gray scale value over the measuringfield.
 4. The method according to claim 3 wherein it is determined thatthe full-tone measuring field is of adequate quality if the uniformitydistribution or the noise is smaller the nominal value.
 5. The methodaccording to claim 3 wherein it is determined that the full-tonemeasuring field is of inferior quality if the uniformity distribution orthe noise is higher than the nominal value.
 6. The method according toclaim 3 further including the step of determining how frequently theactual value exceeds the nominal value of the uniformity distribution orthe noise.
 7. The method according to claim 2 wherein when the measuringfield consists of a halftone measuring field for a printing ink and theactual value for the halftone measuring field is determined in the formof a geometric parameter for the halftone measuring field.
 8. The methodaccording to claim 7 wherein it is determined that a halftone measuringfield is of adequate quality if the geometric parameter is lower thanthe nominal value.
 9. The method according to claim 7 wherein it isdetermined that the halftone measuring field is of inferior quality ifthe geometric parameter is higher than the nominal value.
 10. The methodaccording to claim 7 wherein the geometric parameter is for a roundhalftone dot of the halftone measuring field and the geometric parameteris determined in the form of a first halftone dot deformation value thatis based on minimum halftone dot diameter and a maximum halftone dotdiameter of the halftone dot.
 11. The method according to claim 10wherein it is determined that doubling of the halftone dot has occurredif the first halftone dot deformation value is higher than the nominalvalue.
 12. The method according to claim 7 wherein the geometricparameter is for a round halftone dot of the halftone measuring fieldand the geometric parameter is determined in the form of a secondhalftone dot deformation value that is based on a minimum halftone dotsurface of the halftone dot and a maximum halftone dot surface of thehalftone dot, wherein the minimum halftone dot surface is determinedwithin a first defined gray scale value range and the maximum halftonedot surface is determined within a second defined gray scale valuerange.
 13. The method according to claim 12 wherein it is determinedthat bleeding of the halftone dot has occurred if the second halftonedot deformation value is higher than the nominal value.